There's no conditional fertility so you'd still probably need an event to abort forbidden pregnancies, but you can fully mod the conditions for who is allowed to marry whom now.
Too bad. I had hoped that as CK3 displays the offspring chance on the marriage screen that you could plug in a script there which checks for racial traits and reduces the offspring chance to 0 accordingly.
How will you do beast/monsterous races?
The mod Lord of the Kings managed to morph human models into lion men and other beast races. I wouldn't say that they are beautiful, especially once DNA drifts after a few generations, but it shows what is possible. They have also more human looking fantasy races like elves and dwarves.
I am curious though how you want to do centaurs as those can not be done my morphing the human model.